This invention pertains to an electronic ballast for powering a lamp, and more particularly to a low-cost electronic ballast having low total harmonic distortion (THD) and crest factor with suitable phase control for dimming and which may also have a high power factor. However, it will be appreciated that the invention may be advantageously employed in related lighting environments and applications.
U.S. Pat. No. 5,994,848 generally discloses an electronic ballast system. A power feedback circuit assists in maintaining a level of current drawn from a triac to at least a level of the triac holding current during load-dimming levels. The feedback circuit is fed into a junction joining together a pair of diodes for converting a high-frequency square-wave signal into a unidirectional signal supplied to an inverter. Overboast voltages across a buffer capacitor are minimized during low-dimming levels.
Although the ""848 patent provides a proposed solution to supplying triac holding current during low-dimming levels, the circuitry required for operation of the cited patent is complex and involved. Particularly, the driving circuit needs to provide a large dead time between switching elements and a lagging/inductive current in its attempt to maintain a zero-switching voltage. Additionally, the circuit described in ""848 over stresses the switching elements at full loads, may under deliver the required holding current at low-dimming levels, and requires the use of a control IC (Integrated Circuit).
Therefore, it is still desirable to develop an electronic ballast that maintains sufficient triac holding current during low dimming levels in an inexpensive, practical manner that is low cost and easily manufacturable. Such a circuit should also be highly reliable.
An electronic ballast is configured to receive the input from a power source in order to control operation of a lamp connected to the electronic ballast. The ballast includes a positive side bus voltage line and a ground reference line. An input section is connected to the power source, to the positive side bus voltage line and the ground reference line. A complementary pair of switches, connected to bus, ground and to resonant network, is controlled by a gate drive network. The gate drive network receives a feedback signal that is coupled to a transformer and controls operation of the set of switches using the received and further processed signals. A triac dimmer is connected between the power source and the input section for providing a dimming capability. A resonant network includes at least a first resonant capacitor connected to the resonant network and to at least one of the positive side bus voltage line and the ground reference line. A second resonant capacitor is connected to the first resonant capacitor and to the input section. At least a portion of resonant current from the second resonant capacitor is applied back to the input section. The portion of resonant current applied back to the input section is sufficient to maintain a minimum triac holding current. In another embodiment of the present invention, a lamp is connected within the ballast circuit such that a portion of its current is also fed to the input circuit.